A controller includes: a casing having a rectangular solid shape and including a front panel including an intake port, a rear panel including a discharge port and opposed to the front panel, and a heat transfer wall including an opening portion and perpendicular to the front panel and the rear panel; a fan inside the casing that sucks outside air into the casing through the intake port and supply the air to the inside of the casing; a regenerative resistor inside the casing at a position where the regenerative resistor is exposed to the outside through the opening portion of the heat transfer wall, the regenerative resistor being insertable and detachable through the opening portion; electronic parts inside the casing and including a servo amplifier which makes the regenerative resistor consume electric power; and a lid body detachably attached to the heat transfer wall to seal the opening portion.